Troops killed 60 militants and seized their largest-ever drugs haul, in a just-ended operation in a Taliban stronghold and opium-production centre in southern Afghanistan, the US military said.

Troops killed 60 militants and seized their largest-ever drugs haul, in a just-ended operation in a Taliban stronghold and opium-production centre in southern Afghanistan, the US military said on Saturday.

The operation in the southern province of Helmand ended overnight when troops used air strikes to destroy their massive haul of 92 tonnes of drugs, heroin-processing chemicals and bomb-making materials, it said in a statement.

Afghan and US-led coalition troops had moved into the Marja area, about 40 kilometres (25 miles) from the Helmand province capital Lashkar Gah, on Tuesday.

"A total of 60 militants were eliminated as they mounted an ineffective and uncoordinated defence against friendly forces," the US military said in its final tally. The previous toll was 47 militants killed by Friday.

It said troops "seized the single-largest drug cache by Afghan-led forces in Afghanistan to date during a four day operation" in Marja.

Troops had moved all "friendly elements" from a bazaar in the area at midnight Friday, it said.

"With the area cordoned off, precision air strikes destroyed the narcotics, bomb-making materials, weapons and munitions discovered during the operation," it said.

Helmand, where thousands of British troops are based for a NATO-led military force helping Afghanistan, is the main producer of Afghan opium, which accounts for more than 90 percent of the world's supply.

Most of it is turned into heroin in the country and smuggled to markets in Europe, Asia and the Middle East.

The vast province is also a stronghold for the Taliban, who were in government between 1996 and 2001 and waging an Al-Qaeda-linked insurgency.